Exam-style question
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Which of the following best describes proper time in special relativity?.
- A.time measured by a clock moving relative to the event
- B.time measured by a clock at rest relative to the event
- C.coordinate time in a particular inertial frame
- D.time measured by a clock moving at the speed of light
Model answer
What a good answer should say
- time measured by a clock at rest relative to the event
Explanation
Why this works
Formula/rule: Δτ = Δt / γ = Δt √(1 – v²/c²). Substitution: For a clock at rest, v = 0 so γ = 1.
Working: Δτ = Δt √(1 – 0) = Δt. Answer: proper time equals the coordinate time measured by a clock that is at rest relative to the events.
Units/conclusion: proper time is measured in seconds and is invariant for that clock.
Common mistake
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